(1) Field of the Invention
The present invention is related to a developing device and an image forming apparatus, and in particular to a technology for keeping an amount of developer constant in a trickle-type developing device regardless of a change in a system speed.
(2) Description of the Related Art
In an image forming apparatus using two-component developer, which includes toner and carrier, when toner is consumed in image formation, fresh toner is supplied as required. However, carrier that stays in a developing device degrades over time. In order to lengthen the lifespan of the developing device, despite the degradation of carrier, a trickle-type developing device has been proposed. The trickle-type developing device supplies fresh carrier while discarding degraded carrier little by little. At first, surface overflow types have been in the mainstream of the trickle-type developing devices. The surface overflow type developing devices employ a configuration having two shafts that are laterally aligned and circulate developer, and discharging carrier when the height of a surface of developer exceeds a predetermined level in the developing device.
However, the surface overflow type developing devices have a problem in that an amount of discharged developer easily varies by being influenced by inclination of the developing device in an axial direction of the shafts thereof. That is, when the developing device is inclined so that the height of one end thereof discharging the developer is low, the amount of discharged developer unnecessarily increases, and as a result, carrier that has not particularly degraded is discarded. To the contrary, when the developing device is inclined in the opposite direction, the amount of discharged carrier becomes insufficient, and as a result, degraded carrier stays in the developing device.
In view of the above problem, for example, a technology for locally narrowing the upper part of a clearance at a reverse flow generating unit located in front of a developer discharge opening for discharging the developer is disclosed (see Japanese Patent Application Publication No. 2005-221852). The reverse flow generating unit is provided with a screw for pushing developer in a direction from the developer discharge opening toward a stirring and conveying tank on a path from the stirring and conveying tank to the discharge opening so as to adjust an amount of developer to be discharged. The stirring and conveying tank is for stirring and conveying developer in the developing device.
By narrowing the upper part of a clearance between the screw and an inner wall of the reverse flow generating unit in a vertical direction, an amount of discharged developer becomes dependent more on a driving power to drive the developer in the stirring and conveying tank than on the height of the surface of developer. Accordingly, the amount of discharged developer becomes not likely to be influenced by inclination of the developing device, and stability of the amount of discharged developer is improved. When the amount of discharged developer is stabilized, the amount of developer in the developing device can be kept constant. Accordingly, an amount of toner supplied to a photoreceptor is stabilized and superior image quality is realized.
On the other hand, circulating overflow types are becoming the mainstream of developing devices with a configuration having two screws that are obliquely or vertically aligned and circulate developer in the stirring and conveying tank, and discharging the developer. Such circulating overflow types determine the amount of discharged developer based not only on the driving power to drive developer but also on a relationship between the driving power and a pushing power from the reverse flow generating unit. Also, since the circulating overflow types are unlikely to be influenced by the height of the surface of developer, such types have advantages of being relatively resistant to the inclination of the developing device and being kept compact in size.
Furthermore, with regard to the configuration having the two shafts that are vertically aligned and circulate developer, a technology for ensuring stability to discharge the developer by making downflow openings smaller than an upflow opening is also disclosed (see Japanese Patent Application Publication No. 2008-250290). Such conventional art is provided with a pair of stirring and conveying tanks that are vertically located, two downflow openings, one upflow opening, and a discharge opening. The developer moves down from the upper stirring and conveying tank to the lower stirring and conveying tank through the two downflow openings, and moves up from the lower stirring and conveying tank to the upper stirring and conveying tank through the one upflow opening. The discharge opening is located downstream from the downflow openings of the upper stirring and conveying tank in a conveyance direction of the developer.
One of the two downflow openings allows the developer to traverse at any time, and the other downflow opening allows the developer to traverse only when the amount of developer in the developing device has increased. In addition, the size of each downflow opening is smaller than the size of the upflow opening. Thus, even when an amount of developer in the developing device is small, degraded carrier is discarded while an amount of developer that moves down is limited, and when the amount of developer in the developing device is large, an amount of developer that moves down increases and accordingly an amount of discarded carrier is prevented from being excessive. As a result, it is possible to stabilize an amount of discarded carrier, which has degraded.
In order to increase productivity (the number of images that are formed per unit time) of an image forming apparatus, a system speed has been increasing. In accordance with the increase of the system speed, increase of a stirring and conveying speed of a developing device is also being required in order to supply a greater amount of developer.
However, as the stirring and conveying speed increases, developer that is conveyed jumps the downflow openings more easily due to inertia forces. Especially, when the downflow openings are made narrow like the above conventional art, the developer jumps the downflow openings even more easily, and accordingly an amount of discharged developer increases. As a result, it becomes difficult to stabilize the amount of discharged developer. Such a problem of instability becomes especially evident when a system speed exceeds 200 mm/second.